We report the observation of molecular ${\mathrm{H}}_2$ chemisorption at the edge sites of a stepped Ni (100) surface that is covered with a dense layer of atomic hydrogen. No such state is observed on the flat Ni (100) surface. We suggest that this ${\mathrm{H}}_2$ chemisorption state can form stably as a result of residual unfilled $d$ states at these step sites, while the dissociative channel is blocked by the saturated atomic adsorbate layer.

• Received 30 May 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.2045